This invention relates to both electrochemical and oxygen bleaching or delignification of lignocellulosic materials particularly wood chips and pulp and more particularly to wood pulp prepared by standard pulping methods, especially alkaline pulping methods, more specifically to retardation of the loss of cellulose viscosity which occurs during such bleaching process, and to products prepared thereby and processes for their use.
Chemical pulp is prepared by treating lignocellulosic material with various "pulping chemicals" to render soluble the major portion of the non-carbohydrate portion of the material. The most common chemical pulp is pulp prepared from wood chips by the "kraft" or sulfate process. In this process the wood chips are treated under heat and pressure with sulfide ions in a strongly alkaline aqueous medium. The resulting pulp, while quite strong, is highly colored probably due to a large number of chromophores in the residual lignin. "White" papers are prepared from such pulps and from other chemical pulps by bleaching which principally comprises further delignification. The usual way this is accomplished is by treatment with chlorine-based chemicals such as chlorine, chlorine dioxide, hypochlorite and other oxidative chemicals which oxidize and solubilize the remaining lignin and, thus, remove the chromophoric material.
Recently other oxidative processes employing materials such as oxygen, ozone, peracids and peroxides have been suggested as alternatives to reduce or replace the need for chlorine based chemicals in the bleaching of pulps. For a number of reasons, well known to those in the art, oxygen has proven to be of particular interest and bleaching sequences employing oxygen which are intended to reduce the use of chlorine based chemicals are in commercial operation. Oxygen is a powerful oxidant. It significantly depolymerizes cellulose in pulp at the same time it is reacting with and solubilizing the highly chromophoric residual lignin. Cellulose destruction is aggravated by the severe reaction conditions (temperatures greater than 90.degree. and oxygen pressures exceeding 70 psi) required for standard oxygen-based bleaching sequences as presently practiced.
Cellulose destruction or depolymerization which is commonly measured by a fall in viscosity as measured by standard pulp viscosity tests, adversely affects the physical properties of paper made from pulp. Reduction of the severity of the conditions employed in oxygen bleaching reduces the tendency towards cellulose destruction. Because conventional oxygen bleaching processes proceed at a rate too slow to be commercially useful under such less severe conditions, one convenient means to use less severe conditions is through the use of catalysts which accelerate the reaction between lignin and oxygen. Several such catalysts are known. They are Salcomine (an ethylenediamine-bis-salicylaldehyde complex of cobalt), ortho-phenanthroline, and manganese salts. These catalysts are not suitable for practical commercial use because they are relatively expensive due to the fact that they cannot be recovered and regenerated conveniently.
One way to generate or regenerate a catalyst for oxygen bleaching is the use of electrochemical treatment of the precursor or spent catalyst, respectively.
Electrochemical generation of oxidants or other "electron carriers" in situ or in a closed cycle process in pulp bleaching and even in some pulping processes for lignocellulosic material has been experimented with in the past but, as far as is known, with little or no practical success and these processes have never been used commercially.
Electrochemically generated compounds such as hypochlorite, hydrogen peroxide and the like have been shown to react with and solubilize lignin. However, compounds lacking an oxygen function, for example ferricyanide, will react with but not solubilize lignin to any practical extent unless some oxygen is also present. The prior art has not recognized the importance of the oxygen that was present in providing its reported results and, hence, has not recognized that compounds such as ferricyanide when present in catalytic amounts together with deliberately added quantities of oxygen function as catalysts to solubilize lignin at a very rapid rate under reaction conditions substantially milder than those employed in conventional oxygen bleaching of lignocellulosic pulps. Oxygen bleaching may, therefore, be conducted under milder conditions of temperature and pressure than are presently employed in conventional processes.
Even under the milder conditions of electrochemically generated ferricyanide catalyzed oxygen bleaching, extensive cellulose destruction still occurs and pulp bleached to a particular kappa level will have a cellulose viscosity level roughly equal to that of a similar pulp bleached by conventional oxygen processes to the same kappa level.
Another convenient method of reducing viscosity loss in oxygen bleaching under the conditions employed in commercial practice is the use of viscosity preservers, principally complexes of magnesium.
It has been found however that under the conditions of electrochemically promoted oxygen bleaching where the temperature is lower than that of conventional oxygen bleaching a substantially completely different group of compounds which are largely ineffective in reducing cellulose viscosity loss in conventional oxygen bleaching are effective in preventing such viscosity loss and enable bleaching to low kappa levels while retaining relatively high cellulose viscosities.